1. Field
The present disclosure relates to a composite thermoelectric material, a thermoelectric element and a module including the composite thermoelectric material, and methods of preparing the composite thermoelectric material.
2. Description of the Related Art
The thermoelectric phenomenon provides reversible and direct energy conversion between heat and electricity. The thermoelectric phenomenon occurs when electrons and/or holes move in a thermoelectric material in response to a temperature gradient.
The thermoelectric phenomena include the Peltier effect, the Seebeck effect, and the Thomson effect. The Peltier effect refers to heat emission or absorption that occurs at a junction of dissimilar materials due to an external current applied to the two dissimilar materials, which are connected to each other by a junction therebetween. The Seebeck effect refers to an electromotive force that is generated due to a temperature difference between opposite ends of the two dissimilar materials which are connected to each other by the junction therebetween, and the Thomson effect refers to heat emission or absorption that occurs when a current flows in a material having a predetermined temperature gradient.
Low temperature waste heat may be converted directly into electricity, and vice versa, using the above-described thermoelectric phenomena. Thus, efficiency of energy utilization may be increased. Also, the thermoelectric material may be applied to a variety of fields such as a thermoelectric generator or a thermoelectric cooler.
Energy conversion efficiency of the thermoelectric material may be represented by a dimensionless figure of merit ZT defined by Equation 1 below:
                    ZT        =                                            S              2                        ⁢            σ            ⁢                                                  ⁢            T                    κ                                    Equation        ⁢                                  ⁢        1            wherein ZT is a figure of merit, S is a Seebeck coefficient, a is an electrical conductivity, T is an absolute temperature, and K is a thermal conductivity.
To increase energy conversion efficiency, a thermoelectric material having a large Seebeck coefficient, a high electrical conductivity, and a low thermal conductivity is desired, but generally, the Seebeck coefficient, electrical conductivity, and thermal conductivity are interrelated and thus have a trade-off relationship.
A nanostructured material has a small particle size compared to a bulk material, and thus an intergranular density of the nanostructured material is greater than that of the bulk material. Accordingly, phonon scattering, which occurs at interfaces, is increased in the nanostructured material. When phonon scattering is increased, thermal conductivity decreases. Further, because the trade-off relationship between the Seebeck coefficient and the electric conductivity collapses due to a quantum confinement effect in a nanostructured material, the figure of merit ZT may be increased through use of a nanostructure.
A nanostructure may be in the form of, for example, a superlattice thin film, a nanowire, a nanoplate, or a quantum dots, but manufacture of the nanostructure is difficult, and a figure of merit ZT of such materials is low in a bulk phase.
Therefore, a bulk material that provides a simple manufacturing process and an increased figure of merit ZT is desired.